A number of hydrocarbon streams produced by refining operations or natural gas processing operations contain mercaptan sulfur compounds and are commonly treated to remove such mercaptan sulfur compounds in order to reduce odor and/or corrosivity associated with these acidic species. For example, alkyl and aryl mercaptans are generally removed from such hydrocarbon streams by washing or contacting such streams with an aqueous solution of an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide. The sulfur compounds are removed into the alkaline stream as mercaptides, i.e. the metal salts of the mercaptans. The alkaline solution containing the mercaptides is then separated from the hydrocarbon stream. In order for the overall treating scheme described to be economically feasible, it is generally necessary to regenerate the alkaline solution and to recycle it for reuse in contacting more of the mercaptan-containing hydrocarbon streams. By "regenerate" is meant the removal of most, if not all, of the mercaptides in the alkaline stream, which may be accomplished by oxidizing the mercaptides to disulfides. The disulfides, which are relatively insoluble in the alkaline solution, may then be removed therefrom as an organic layer. In order to carry out the oxidation, a catalyst is generally employed. Any space suitable catalyst known to those skilled in the art may be utilized, including, for example, any of the catalysts disclosed in U.S. Pat. No. 3,574,093, such as cobalt phthalocyanine or a derivative thereof, such as cobalt phthalocyanine disulfonate.
A typical process for conventionally regenerating the alkaline stream involves the use of an oxidation zone, which typically involves a column having suitable contacting means such as trays with bubble caps, or suitable packing material such as Raschig rings and the like. The alkaline stream in the column is contacted with air in the presence of the oxidation catalyst which is generally contained in the alkaline stream to be regenerated, in order to oxidize the mercaptides to disulfides. Generally, such oxidation zones are relatively large columns which are very expensive to construct and maintain. After a spent alkaline mercaptide containing stream has been processed through an oxidation zone, there is obtained a mixture of regenerated alkaline solution and disulfides, which mixture is typically in the form of a fine dispersion. This mixture must be introduced into a settling zone wherein the mixture is allowed to reside for a considerable time in order to enable the dispersion to coalesce and separate into layers. Due to the fineness of the dispersion, the settler must be relatively large and occasionally additional coalescing aids, mechanical and/or chemical, may be required to insure separation of the disulfide and alkaline phases. There are also systems where the regenerated caustic disulfide mixture is contacted with a solvent solution as is employed with other hydrocarbon treating systems in order to effectively deal with this separation problem.
Another conventional regeneration process consists of air/steam stripping the mercaptides from the alkaline stream, a process which also requires substantial equipment and additionally is energy intensive.
As previously discussed, a settling tank is required in conventional practice to process the dispersed disulfide-caustic mixture by allowing the two phases to coalesce and to separate. The problems of separating constituents of a dispersed or emulsified mixture of this nature necessitate equipment substantial in cost and size. The overall removal of sulfur compounds from alkaline streams thus is limited by several factors, including the capacity of the equipment which is reflected in the efficiency of the oxidation of mercaptides to disulfides and the cleaness or sharpness of the separation of the resulting disulfide or disulfide/solvent mixture from the caustic. Then, of course, there is the increased size of equipment used in these processes with their attendant higher installed cost and higher operating costs, which are additional disadvantages.
It is conventional practice to treat various hydrocarbon streams containing mercaptan sulfur compounds by contacting such a stream with an alkaline solution such as aqueous sodium hydroxide (caustic) whereby the mercaptans are absorbed into the caustic and reacted with it to form mercaptides and thus are separated from the hydrocarbon stream. It is also conventional practice to thereafter regenerate the mercaptide containing caustic solutions to remove the mercaptides and thus render the caustic solution suitable for reuse. The various methods used to regenerate the caustic typically do so by oxidizing the mercaptides to disulfides, generally in the presence of an oxidation catalyst such as certain metal chelates, including, for example, cobalt phthalocyanine disulfonate. Such processes produce a mixture of regenerated caustic and disulfides which must be separated before the regenerated caustic can be reused. Typically, the oxidation of mercaptides to disulfides and the separation of the regenerated caustic from the disulfides is accomplished in separate steps. Also, the separation is typically not complete, i.e. excessive disulfides remain in the caustic solution and regenerated caustic remains in the disulfides. These shortcomings limit the usefulness of the regenerated caustic for further extraction of sulfur compounds from the hydrocarbon stream in question in that the entrained disulfides (sulfur compounds) can be extracted back into the hydrocarbon stream defeating the purpose of treating the hydrocarbon stream with caustic, which is to remove sulfur compounds.
U.S. Pat. No. 2,921,021, Urban, et al, relates to the treatment of sour hydrocarbon distillate with an alkaline solution. The spent alkaline solution containing mercaptides is then mixed with air in a regenerator whereby the mercaptides are oxidized to disulfides. The regenerated caustic and disulfides are in the form of a finely dispersed mixture. The dispersion is passed through a coalescing system and then to a settling tank whereby the disulfide compounds are separated from the alkaline solution. While most of the disulfides are removed in the settling tank, in some cases the settling step may be followed by a naphtha wash to remove disulfides still retained in the alkaline solution.
U.S. Pat. No. 2,853,432, Gleim, et al., discloses the regeneration of used alkaline reagents by oxidizing same using a phthalocyanine catalyst. For example, mercaptides contained in a caustic solution were oxidized to disulfides, which were then withdrawn from the regeneration zone by skimming or by dissolving in a suitable solvent such as naphtha.
U.S. Pat. No. 3,574,093, Strong, relates to a multi-step process wherein the spent caustic generated by treating a low-boiling hydrocarbon stream for mercaptan removal is thereafter used in a second treating step wherein a higher boiling sour distillate is sweetened. In the sweetening step, the mercaptans in the sour distillate are oxidized to disulfides. The disulfides exit the treating stage in the hydrocarbon stream along with those mercaptides which had been previously extracted from the low boiling hydrocarbon stream. Thus, the higher boiling stream is sweetened and the partially spent alkaline stream is regenerated at the same time. The regenerated caustic is then introduced into a separation zone from which the disulfide phase is recovered from the caustic zone. The coalescence of the disulfide compound into a separate phase is stated to be extremely difficult without the use of coalescing agents. In addition, a high residence time is used in the separation zone to further facilitate this phase separation.
U.S. Pat. No. 4,362,614, Asdigian, also relates to a multi-step process for the extraction of mercaptans from hydrocarbon streams with an alkaline solution, followed by the regeneration of the mercaptide-containing alkaline solution resulting from such extraction by oxidation in the presence of a catalyst in an oxidation zone, followed by the separation of the disulfides and the alkaline solution by decantation within a phase separation zone. From this process, the alkaline solution is recycled. In addition to the requirement of a separate oxidation zone and a large settling zone, the use of additional coalescing means is said to be required.
U.S. Pat. No. 3,758,404, Clonts; U.S. Pat. No. 3,977,829, Clonts; and U.S. Pat. No. 3,992,156, Clonts, are directed to methods and apparatus for liquid-liquid mass transfer between immiscible liquids. A first liquid is introduced onto the upstream surface portion of a plurality of fibers extending generally along and secured within a conduit. A second liquid, immiscible with said first liquid, is flowed through the conduit cocurrently with the first liquid, thereby dragging a film of the first liquid along the fibers. The two liquids are collected at the downstream end of the conduit in a collection vessel or gravity separator. By this technique, a large surface area is generated between the two liquids and mass transfer between the two liquids is facilitated. As a result, a component of either of the liquids may be transferred either into or out of the liquid film as it moves along the fibers. The mass transfer may also occur as the result of a chemical reaction at the interface between the liquids, such as the removal of acidic constituents from a hydrocarbon by reaction with a base in an aqueous solution, or the transfer may be without a chemical reaction, such as by extraction from one liquid to another. The patents teach the introduction of an aqueous caustic solution onto the fibers and the flowing of gasoline containing acidic components cocurrently therewith. The acidic components of the gasoline react with and are absorbed by the caustic.